1. Field of the Invention
The present invention relates to a hot-parison type injection stretch blow molding method and apparatus which continuously perform at least steps of injection molding, blow molding and ejecting a preform (parison) throughout one stage.
2. Description of the Related Art
A typical one-stage blow molding apparatus of such a type is disclosed in Japanese Patent Publication No. Sho 58-22096. The apparatus comprises a machine base which has an injection molding station, a temperature regulating station, a blow molding station and an ejecting station respectively located on the base at its four sides. The apparatus also comprises a rotary disc rotatably disposed above the base. The rotary disc includes four neck plates disposed on the rotary disc at its four locations. The neck plate supports a plurality of neck molds each for holding a neck of a preform or bottle. When the rotary disc is intermittently rotated every 90 degrees, the four neck plates are sequentially moved to the respective stations.
Japanese Patent Laid-Open No. Hei 1-275122 describes an improvement on the aforementioned apparatus to increase the efficiency of production per unit time. In the improved apparatus, two lines of neck plates are disposed on the rotary disc at each of the four locations therein.
Japanese Patent Publication No. Sho 64-8657 describes a system for conveying a plurality of neck mold moving units along linear path, rather than the rotary disc. The system comprises injection molding, temperature regulating and blow molding stations which are arranged in a line. Each of the neck mold moving units is asynchronously moved with the injection molding cycle to facilitate the setting of the molding time.
In this connection, the efficiency of bottle production per unit time in the blow molding, apparatus depends on the working ratio of blow molds in the blow molding station. It was pointed out that the aforementioned one-stage type blow molding apparatus is lower in efficiency than two-stage type blow molding apparatus. The two-stage type blow molding apparatus comprises one stage of injection molding a preform and another separate stage of blow molding a bottle which is formed from the preform in the injection molding stage. The two-stage type blow molding apparatus can supply a preform to the blow molding station synchronously with the blow molding cycle and thus realize a relatively high production efficiency. In the one-stage blow molding apparatus, however, the molding cycle time depends on time required to perform the injection molding process, which is longer than time required to make the blow molding process. It is thus impossible to supply the preform synchronously with the blow molding cycle time. As a result, the working ratio of the blow molds in the blow molding station could not be improved.
In Japanese Patent Laid-Open No. Hei 3-159726, the inventors has proposed a system for conveying the neck mold moving units asynchronously with the injection molding cycle to improve the working ratio in the blow molding station.
The proposed system comprises a working line of rectangular configuration including preform injection molding, temperature regulating, blow molding and ejecting stations which are arranged on the working line along its one longitudinal side. The other longitudinal side of the working line is used to return the neck mold moving units to the injection molding station. The system includes two lines of injection molding stations, two lines of temperature regulating stations and one line of blow molding station. Preforms respectively injection molded and temperature regulated by the injection molding and temperature regulating stations can be aligned into a single line and then conveyed into the blow molding station. Thus, the working ratio of blow molds in the blow molding station can be improved.
However, if preforms are simultaneously molded in both the two injection molding stations, preforms from one of the injection molding lines can be conveyed directly into the blow molding station only when another preforms from the other injection molding line is in its stand-by state. In the hot-parison type blow molding apparatus, the blow molding is carried out utilizing the potential heat provided by the injection molding step. Therefore, the temperature in the stand-by preform may vary through the passage of time. It becomes impossible to form bottles of uniform quality throughout the lines.
Thus, the prior art cannot blow mold preforms under equal conditions after they have been injection molded and temperature regulated in the different lines, respectively.
In the prior art, furthermore, the neck mold moving units must be returned from the ejecting station back to the injection molding stations through a relatively long distance therebetween. The neck mold moving units must be held at their stand-by position in front of each of the injection molding station lines. It is therefore required to provide at least two neck mold moving units more than the total number of stations.
The neck molds used in the one-stage type blow molding apparatus are also used as neck cavity molds in the injection molding stations. Thus, the neck molds requires a substantially increased dimensional accuracy and are expensively manufactured. As a result, the increased number of neck molds will increase the manufacturing cost of the entire system.